How did console flexowriters connect to their computers?

Question

Several early computers (e.g., KDF9, LGP30) used Friden Flexowriters for console functions.

What was the nature of the connection between the Flexowriter and the computer? Was this something we would recognize as a "serial interface" today?

As well as the usual keyboard and printer functions, Flexowriters typically had integrated papertape readers and punches. Were these controlled by inband signaling (like XON/XOFF for ASCII Teletypes; not that I'm saying a Flex used ASCII) or "extra wires"?

Flexowriters also seemed to have switches, some of which were clearly for local device control (start tape, perhaps) but others seem to be computer control (the LGP30 Flex had what looks like a "start computer" switch). How did they communicate with the computer?

If any answer is computer-specific, please indicate the computer you're talking about.

Answer 1

Partial answer:

While I wasn't able to find schematics for how the Flexwrite is wired up to the KDF9 (say), there are some Flexwriter manuals availavle on bitsavers. The Flexwriter apparent came with 5 bit and 8 bit encodings (or "5-channel" and "8-channel" variants, as they call them).

The encodings (at least the earlier ones used for the KDF9) were Friden-specific, and neither ASCII nor compatible to anything else. However, the 5-bit encoding worked similar to other Teletext-like encodings (there are shift-states for upper and lower case, and the "all holes punched" encoding is a null encoding).

The Communications System and Equipment (1961) brochure suggest that serial communication was in use, also over longer distances. (Though that doesn't exclude the possibility the Flexwriters connected directly to the computer used a parallel connection).

There is no indication of any software flow control (XON/XOFF). There is also no indication of character codes which could be used to change channels etc. as part of the transmitted data, so I'd assume this could only be done on the flexwriter.

In various materials it is mentioned that the Flexwriters operated at a fixed speed.

There is also plenty of material on the KDF9. This here describes the I/O system as using DMA, that's probably why software flow control wasn't necessary.

It also mentions that there was a dedicated Flexwriter used as the console for the KDF9 which had an "attention" key that was hardwired as an interrupt source, and this feature seems to have been unique for this special Flexwriter. So that was probably a custom modifications.

All I/O channels of the KDF9 work similarly, so likely other Flexwriters were just hooked up to a single channels like the console.

Edit

Looking at the PDP-1 Maintenance Manual, it says on page 4-4

(c) The typewriter logic cable is equipped with a 50-pin Cannon connector

and on page 9-10 ff.

The PDP-l typewriter is an IBM Model B equipped with a Soroban electromechanical encoder and decoder. The decoder contains six information solenoids. These solenoids are driven from the typewriter control unit. The positions of the solenoid armatures are mechanically decoded to determine the desired character. Besides providing the six information signals, typewriter control also energizes the typewriter cam magnet, thereby causing the typewriter to print the desired character. The typewriter decodes most control characters (carriage return, backspace; space, tab, and shift), and all print characters, from the six information solenoids. The two color characters, red and black, are decoded by the typewriter control unit, which directly controls the color shift solenoid in the typewriter.

When the operator strikes a typewriter key, the typewriter encoder presents information signals to the typewriter control unit. The encoder mechanically encodes all print characters into six coded switch closures plus a common switch closure. Control characters are not encoded at the typewriter. When a control character key is struck, a single switch is closed. Closure of such a control char'acter switch is encoded within typewriter control.

So this looks very much like a parallel connection, very likely 5 bits plus parity.

Answer 2

Keep in mind, you're talking about an age where no computer and even less it's devices where standardized components. Every manufacturer has his own ideas and parts where modified to fit.

And the Flexowriter did perform great here, as it was quite configurable, in some way even programmable by selecting/exchanging the parts. Hence the Flex part :))

---

What was the nature of the connection between the Flexowriter and the computer? Was this something we would recognize as a "serial interface" today?

Possible, depending on the computer, as there was also 6 bit parallel interface possible, but in most cases it would have been serial.

As well as the usual keyboard and printer functions, Flexowriters typically had integrated papertape readers and punches. Were these controlled by inband signaling (like XON/XOFF for ASCII Teletypes; not that I'm saying a Flex used ASCII) or "extra wires"?

Yes :))

As before. You have to look at the computer they where made for. Most would use some specific protocol to enable or disable the keyboard.

Flexowriters also seemed to have switches, some of which were clearly for local device control (start tape, perhaps) but others seem to be computer control (the LGP30 Flex had what looks like a "start computer" switch). How did they communicate with the computer?

Usually via separate wires.

If any answer is computer-specific, please indicate the computer you're talking about.

Lets skip that, as it would be an endless list with entries for next to all comuters using a Flexowriter.

Answer 3

I'm adding my own answer as a sort of summary of what I got from the other two excellent answers.

1. It depends on the computer, though the evidence favours a parallel interface for tightly-integrated consoles.

2. Auxiliary device control, and computer control functions, were conveyed by specific signals rather than (say) in-band control characters.

I'd like to "accept" both the previous answers, but alas...

Answer 4

For the Librascope/Royal Precision/General Precision computers, i.e. the LGP-30 and its little and big brothers (LGP-21 and RPC-4000), the actual data transmission is 6 bit parallel. Two sets of 6 data lines, to and from the Flexowriter respectively.

This is complemented by a handful of handshake and sync signals, to indicate that characters are available, trigger their processing, or signal that an "end of input" code has been entered by the user or read by the paper tape reader. Some of the Flexowriter's handshake signals are directly derived from its mechanical camshaft. So they expose an amount of machine detail to the computer which is unexpected by today's standards.